1. Field of the Invention
The present invention relates to a forging method and forging apparatus enabling improvement of the precision of a forged article and reduction of the cost of production.
2. Description of the Related Art
In the past, the method of shaping a forging use material which imparts pressing force to a material made of a metal so as forge the material into a predetermined shape has been known.
Further, in the general forging method, obtaining a forged article of the desired dimensional precision is difficult, so the forged shaped article is finished by machining it in a separate process to give it a desired dimensional precision. When performing this additional machining to secure the desired dimensional precision, there is the problem of that the machining increases the number of production processes.
To solve this problem, there is the technique of ironing the outer circumferential portions of a workpiece to improve the dimensional precision of the outer circumferential portions (see Japanese Patent Publication (A) No. 2004-3449311). However, when making the material of the end face of the workpiece flow to forge it into recessed/projecting shapes, in particular when forming a groove shape not point symmetric with the end face of the workpiece (hereinafter referred to as a “non-point symmetric shape”) (see FIG. 3), the thickness becomes uneven, so it becomes hard to make the internal stress at the different positions inside the material of the workpiece uniform. In this case, underfill and other defects occur. Further, the walls of the groove are not formed vertical, but end up being formed at a slant, so there were also the problems that the desired precision could not be secured and the required performance could not be satisfied without machining.
This problem will be explained in detail. FIGS. 3A to 3C are views showing a shaped article obtained by shaping a workpiece, wherein FIG. 3A is a view of the shaped article as seen from the X-direction, FIG. 3B is a cross-sectional view of a shaped article, and FIG. 3C is a view of the shaped article as seen from the Y-direction. FIG. 4 is an enlarged cross-sectional view of a groove part of a shaped article. FIG. 6 is a view showing the precision of a groove part of a shaped article forged by a conventional forging method based on actual measurement data. FIG. 8 is a view of the appearance of a workpiece worked by a conventional method. FIG. 9 is a view for explaining the conventional forging method and a view in the state in the middle of plastic deformation of the workpiece.
As shown in FIGS. 3A to 3C, the shaped article 90 is for example an automobile brake part and forms a substantially columnar shape overall. It has a groove 93 and outside wall part 91 having a “non-point symmetric shape” with respect to the axial center Z of the column. That is, the substantially circular shaped curves of the groove walls 93a and 93c form “non-point symmetric shapes”, so do not overlap with the original substantially circular shaped curves when rotated halfway about the axial center Z of the column. That is, the center of the substantially circular shaped curve of the groove wall 93a or 93c is offset from the axial center Z of the column. By the formation of the groove 93, an elliptical island part 94 is formed at the center, while an embankment shaped outside wall part 91 is formed at its outer circumference. The outside wall part 91 has a wide part (thick part) 91a and narrow part (thin part) 91b. 
On the other hand, in FIG. 9, 50 shows a conventional shaping apparatus. 51 indicates a top punch forming the die top part, 52 indicates a bottom punch forming the die bottom part, 3 indicates a die, W indicates a workpiece before forging (end face of workpiece before forging shown by one-dot chain line), W1 indicates the workpiece in the state in the middle of the forging (shown by solid line), and C indicates a die cavity for forming recessed/projecting shapes at the end face of the workpiece W. Note that the top punch 51 and bottom punch 52 are substantially identical members, while the cavities C of the top punch 51 and the bottom punch 52 are shaped the same.
The top punch 51 and bottom punch 52 are assembled slidable with respect to the die 3. The bottom punch 52 and die 3 are fastened to the body of the forging apparatus (not shown) and will not move. When a workpiece W is set on the bottom punch 52, the top punch 51 is inserted into the center hole 3a of the die 3 and set right above the workpiece W. Next, the top punch 51 presses the workpiece W by a drive apparatus (not shown) by a load P0 to move it downward in the axial direction. The top punch 51 moves downward to a predetermined position, then rises. In this way, the shape of the bottom end face of the top punch 51 (including cavity C) is transferred to the top end face of the workpiece W.
When the top punch 51 starts to press the workpiece W, the material of the two end faces of the workpiece W is plastically deformed by the pressing forces of the projection 51a of the top punch 51 and the projection 52a of the bottom punch 52 and flows into the cavity C. At this time, the shape of the workpiece W is expressed by W1. Note that the width of the projection 51ax of the top punch 51 is wider than the width of the projection 51ay, while the width of the cavity C2 part is wider than the width of the C3 part.
As shown in FIG. 9, the amount of flow of the workpiece W into the wide width cavity part C2 is large, while the amount of flow of the workpiece W into the narrow width cavity part C3 is small. This is due to the facts that the resistance to the flow of the material to the wide width cavity part C2 is smaller than C3 and that the width of the projection 51ax is wide, so there are many parts of the workpiece material receiving the pressing load. Further, the portion of the workpiece abutting against the projection 51ax becomes high in internal stress. For this reason, at the narrow width cavity part C3, the workpiece material does not fill the inside of the cavity and underfill, where part of the portion corresponding to the workpiece after forging is missing, easily occurs.
As shown in FIG. 8, it is learned that in a shaped article obtained by a conventional forging method, the arrow part becomes too thin. Further, the wall 93ay at the island part 94 side corresponding to the narrow groove part 93 is not formed vertically but is formed at a slant. According to experiments, in a shaped article of a diameter of 40 mm, a height of 30 mm, and a groove depth of 5 mm, the maximum amount of slant relating to the wall 93ay was about 100 μm (see FIG. 6). Note that the required target value of this maximum amount of slant is within 50 μm. In a shaped article made by the conventional method, the required target value was not achieved.
On the other hand, FIG. 4 is an enlarged cross-sectional view of a groove part 93 of a shaped article 90. Further, FIG. 6 is a view expressing the precision of a shaped article forged by a conventional method based on actually measured data. FIG. 4 is a view for explaining the definitions of the abscissa x and ordinate y of FIG. 6 (and later explained FIG. 5). In FIG. 4, s is the reference point, and t is the measurement position. Further, x indicates the distance from the reference point s at the measurement position t, while y indicates the amount of slant of the groove wall 93a from the reference point s at the measurement position t. As shown in FIG. 6, the maximum amount of slant of the groove wall of the location B corresponding to the thin part 91b (see FIG. 3(a)) was, by actual measurement value, about 100 μm, while the maximum amount of slant of the groove wall of the location A corresponding to the thin part 91a was about 60 μm. It will be understood that the location A and location B greatly differ in amount of slant of the groove wall. Note that the target value of the maximum amount of slant of the groove wall is 50 μm.